Cathode ray tubes are used extensively in a variety of display applications including direct view and projection television sets, monitors for computer terminals, television and avionics systems, etc. In many applications (such as projection tubes), high image brightness is required which can only be obtained by the use of a very high power density electron beam. Such high power densities often degrade conventional cathode ray tubes and therefore limit the lifetime of high intensity cathode ray tubes.
A significant advance in the development of high intensity cathode ray tubes was the discovery that certain luminescent epitaxial garnet films on single crystal substrates could withstand much higher power densities than with polycrystalline phosphors without tube degradation (see, for example, J. M. Robertson et al., Applied Physics Letters, 37(5), pp. 471-472, Sept. 1, 1980). Several systems were examined using yttrium aluminum garnet as the substrate and various activators in yttrium aluminum garnet in the epitaxial layers. The activators examined were Tb, Eu, Pr, Tm and Ce. The epitaxial layers were grown by liquid phase epitaxy using a PbO--B.sub.2 O.sub.3 flux.
These types of fluorescent screens did indeed withstand much higher electron power densities than conventional screens and maintained their performance without long term degradation. Because of the magnitude of the index of refraction (1.84 at 550 nm for yttrium aluminum garnet), a large fraction of the light generated within the single crystal luminescent screen is trapped by internal reflection. It is highly desirable to increase the fraction of generated light extracted from the single crystal screen.
Various proposals have been made to increase the amount of light exiting the single crystal cathode ray tube. One proposal, described by P. F. Bongers et al. in U.S. Pat. No. 4,298,820 issued on Nov. 3, 1981, has a surface epitaxial layer on the luminescent screen with V-shaped grooves. These grooves increased the amount of light exiting the tube. The pattern of grooves was produced by an etching procedure on the epitaxial film, but such a procedure is difficult to carry out on such chemically stable crystals as yttrium aluminum garnet. An alternative procedure, described by I. F. Chang et al., IBM Technical Disclosure Bulletin, 25(5), 1982, page 2630, involves the immersion of the garnet layer in a fluxed melt at a temperature above the saturation temperature of the fluxed melt. This procedure leads to dissolution of part of the epitaxial layer to form a facet surface structure. An alternative approach is to form facets on the single crystal substrate via etching prior to the growth of the luminescent layer.
Single crystal cathode ray tubes would be much more attractive commercially if a simple, easily made structure could be devised to couple light out of the structure. Particularly attractive would be a process which was easy, reliable, and attractive from a commercial fabrication point of view. In particular, the surface structure of the tube and process for making the tube should not induce detrimental mechanical defects in the cathode ray tube, adversely affect the cathodoluminescence efficiency, and preferably be applicable to a wide variety of crystal orientations and material systems.